1. Field of the Invention
The present invention relates generally to the hydrometallurgical recovery of metals from their source materials. More particularly, the present invention relates to the hydrometallurgical recovery of copper from its oxidized ores by utilizing alkaline leaching agents such as strong hydroxide or carbonate solutions.
2. Description of the Prior Art
It is presently known that copper may be recovered from its oxidized ores by utilizing an acid leaching agent such as sulfuric acid. However, the use of an acid leachant is feasible only when the oxidized copper ores are relatively free of calcium and/or magnesium carbonates. There are substantial amounts of oxidized copper ores which contain calcium and/or magnesium carbonates in such large quantities that the use of an acid leach is unsuitable because the acid reacts with the carbonates and largely exhausts itself thereon before combining with the copper. Other known leaching agents, such as cyanide and ammonia, have been utilized but these agents have been found to be objectionable primarily because of their toxicity and difficulty in handling.
In the situation where the copper is to be recovered from oxidized ores containing large amounts of carbonates, it has been proposed that such ores be leached with an alkaline or caustic leach solution such as an alkali metal hydroxide or carbonate. It has been shown that the higher solubilities of the hydroxides permit the use of stronger and consequently faster acting solutions. As such, the hydroxides are therefore preferable to the carbonate leach solutions.
The use of strong base leaching solutions, such as those containing sodium hydroxide, are known to be capable of solubilizing the copper contained in such minerals as azurite, malachite and chrysocolla. The species in solution is presumed to be some form of a hydroxy copper (II) complex. However, it has been found that difficulties in the form of erratic and low recovery rates occur when sodium hydroxide solutions are used to extract copper from oxidized copper ores. Though the basic causes of such difficulties have not heretofore been identified and overcome, the industrial implementation of sodium hydroxide leaching has been inhibited, if not abandoned altogether, notwithstanding the recognized advantage of this basic technique.
The present invention has discovered that one reason for the poor and erratic recovery of copper from its oxidized ores during alkali hydroxide leaching, such as with a sodium hydroxide solution, is the dehydration of the hydroxy copper (II) complex. This dehydration results in reprecipitation of the copper values in the slurry and, consequently, poor extraction. This undesirable effect is further promoted by higher temperatures and the presence of certain minerals in the ore itself.